During their evolution, many marine organisms have developed mechanical defense mechanisms to protect themselves against marine bacteria. The same marine bacteria which so swiftly dispatch the remains of dead organisms represent an ever present threat to the living organism. Hence, many marine organisms must constantly produce antibacterial substances to ensure their survival. In particular, the more primitive marine organisms appear to rely heavily on a chemical defense against bacterial infection. We therefore propose to search for antibacterial substances from marine organisms, particularly sponges, marine bacteria and algae. We have isolated several antibiotic-producing marine bacteria and have found seven antibiotic substances from two species of marine bacteria. We have synthesized the sponge antibiotic aeroplysinin-1 and some related compounds.